Stator blade for a turbomachine, especially a steam turbine

ABSTRACT

The invention relates to a stator blade ( 4 ) of a turbomachine, especially of a steam turbine, which has the following geometric features. A lean curvature, a swept curvature, a twist in the radial direction of the respective blade ( 4 ), a hub-side circumferential step ( 14 ) which in the direction of flow ( 15 ) falls away inwards and radially to the rotational axis ( 8 ) of the turbomachine, a chord length (s) of the blade which varies over the radial extent of the stator blade ( 4 ), and also a cross-sectional profile of the blade ( 4 ) which varies over the radial extent of the stator blade ( 4 ).

TECHNICAL FIELD

The present invention relates to a stator blade for a turbomachine, especially for a steam turbine with at least one stator blade row.

BACKGROUND OF THE INVENTION

Curved blades are used especially in steam construction as an embodiment of turbine blades particularly when strong three-dimensional flows occur, which feature pronounced radial differences in the static pressure variation between rotor side and stator side, and which arise as a result of the deflection in the stator blades. The flow of a flow medium in a last stage of a low-pressure turbine with large inflow cross section leads to a radial reaction distribution which acts negatively upon the efficiency of the steam turbine, especially in the case of a large ratio between blade length and hub. The reaction distribution in this case is different in the radial direction, wherein the reaction distribution is low at the hub and high in the region of a casing of the turbine, which is generally regarded as being disadvantageous.

A high reaction in the hub region reduces the gap losses in the stator blade ring and therefore leads to improved efficiency. In order to optimize the radial reaction distribution, curved stator blades are therefore used.

A turbine with stator blades which are curved only in the circumferential direction is known from DE 37 43 738 A1, the blade curvature of which is directed over the height of the blade towards the pressure side of the adjacent stator blade in the circumferential direction in each case. Blades, the curvature of which is directed over the height of the blade towards the pressure side of the adjacent stator blade in the circumferential direction in each case, are additionally known from the aforesaid publication. Consequently, boundary layer pressure gradients which extend both radially and in the circumferential direction are to be reduced in an effective manner and as a result the aerodynamic blade losses are altogether reduced.

Turbines with stator blades which are curved in the axial direction and in the circumferential direction are known for example from DE 42 28 879 A1. Upstream of a rotor cascade, in this case a fixed stator cascade is arranged, the rotor blades of which are fluidically optimized for full load with regard to number and also with regard to their chord-to-pitch ratio. The stator blades impart to the flow the swirl which is necessary for entry into the rotor cascade. The curvature of the blades extends perpendicularly to the chord, which is achieved as a result of displacement of the profile cross section both in the circumferential direction and in the axial direction. The curvature of the stator blades is directed towards the pressure side of the adjacent stator blade in the circumferential direction in each case. As a result of this curvature perpendicularly to the blade chord, the blade surface which is projected in the radial direction is greater than in the case of a known curvature only in the circumferential direction, as a result of which the radial force upon a flow medium is increased so that the flow medium is pressed onto a passage wall and reduces the boundary layer thickness there.

A turbine blade is known from WO 2005/005784 A1, which in the direction of flow is negatively swept on its rotor-side end and on its stator-side end, and in a direction which is radial with regard to the direction of flow is inclined towards the pressure side on its rotor-side end and also on its stator-side end. In this case, therefore, it concerns a turbine with turbine blades which are curved both in the circumferential direction and in the axial direction.

A final stage of a turbine which is exposed to axial throughflow, with a large passage divergence and also with a row of curved stator blades and a row of tapered and twisted rotor blades is known from EP 0 916 812 B1, wherein the stator blades in the axial direction are positively swept on their rotor-side end and negatively swept on their stator-side end, in each case with regard to the run of the rotor-side passage boundary. The positive sweep of the stator blade in this case extends over two thirds of the height of the blade and then merges into the negative sweep, wherein in the region of positive sweep the stator blade trailing edge extends parallel to the stator blade leading edge, and in the region of negative sweep an axial diffuser, which continuously widens towards the wall, is formed between stator blade and rotor blade with increasing deceleration of the axial component of the flow medium.

Further turbines with turbine blades which are curved in the circumferential direction and/or in the radial direction are known for example from U.S. Pat. No. 5,249,922, from U.S. Pat. No. 4,470,755, from U.S. Pat. No. 4,500,256 or from EP 0 425 889 A1.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a stator blade for a turbomachine, which by reducing the aerodynamic blade losses enables an improved efficiency of the turbomachine to be achieved.

This problem is solved by means of the subject of the independent claim. Preferred embodiments are the subject of the dependent claims.

The invention is based on the general idea in the case of a turbomachine of providing at least the stator blades of a stator blade row with a lean curvature, a sweep curvature, a twist, a chord length which varies over the radial extent of the stator blade, and a cross-sectional profile which varies over the radial extent of the stator blade. In addition, the stator blade row has a hub-side circumferential step which in the direction of flow falls away inwards and radially to the rotational axis of the turbomachine. As a result of this, a number of advantages can be combined. On the one hand, a radial distribution of a mass flow which flows through the turbine and also a radial pressure gradient are reduced, while on the other hand a greater mass flow, that is to say, throughflow volume, is induced in the region of the hub. At the same time, the impingement energy of water droplets is reduced, as a result of. which the erosion behavior is favorably influenced. In particular, the reduced impingement energy can be used for reducing the reaction degree at the blade tip, as a result of which lower absolute velocities can be realized on a stator-blade trailing edge so that lower leakage losses are encountered.

Further important features and. advantages of the stator blade according to the invention for a turbomachine result from the dependent claims, from the drawings and from the associated figure description with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are represented in the drawings and are explained in more detail in the following description.

In this case, in the drawing, schematically in each case,

FIG. 1 shows a cross section through a turbomachine according to the invention in the region of a stator blade,

FIG. 2 shows a longitudinal section through the turbomachine in the region of a stator blade,

FIG. 3 shows a plan view of a stator blade in the radial direction,

FIG. 4 shows a longitudinal section through the turbomachine in the region of a hub-side step,

FIG. 5 shows a much schematized view for illustrating a pitch ratio,

FIG. 6 shows a view as in FIG. 5, but for illustrating a wedge angle.

DETAILED DESCRIPTION OF THE DRAWINGS

According to FIG. 1, a sectioned stator blade 4 is exemplarily shown in a flow space 1 which is arranged between a rotor hub 2 and a radial outer wall 3, that is to say the casing. The statement that in this case it concerns a stator blade 4 is not to be interpreted with limitation so that other blades, such as rotor blades, which are arranged in turbomachines, are also to be covered by the invention.

As shown in FIG. 1, the stator blade 4 has a so-called lean curvature which is directed in the circumferential direction, and wherein a curvature angle y varies along the radial blade length, that is to say from the hub 2 towards the radial outer wall 3. In the case of the embodiment which is shown in FIG. 1, the lean curvature of the stator blade 4 decreases along the radial blade length from the blade root, that is to say from the hub 2, towards the blade tip, that is to say towards the outer wall 3. In the case of the lean curvature of the stator blade 4 it is a positive lean curvature, i.e. the curvature extends in the direction of rotation 5 of the stator blade 4. The shape of the curved stator blade 4 in this case preferably represents a generally continuous arc which forms an acute angle γ with the hub 2 or with the outer wall 3. The curvature angle γ lies between a tangent 7 which lies against a blade surface 6 at a trailing edge 12 or at a leading edge 16 of the stator blade 4, and a radial line 9 which extends orthogonally to the rotational axis 8 of the turbomachine and lies preferably within a range of 0°≦γ≦15°.

In FIG. 2, a so-called sweep curvature of the stator blades 4 is shown, by which is understood a curvature in the axial direction, i.e. parallel to the chord 10 of the stator blades 4. The sweep curvature in this case is described by a curvature angle δ which varies along the radial blade length and has a positive value at the hub 2 and a negative value in the region of the casing 3. A positive value in this case is defined according to FIG. 2 by the chord 10 extending above a point of intersection 11 with the radial line 9 which extends orthogonally to the rotational axis 8 of the turbomachine, to the right of the radial line 9, while in the case of a negative curvature angle δ the chord extends above the point of intersection 11 to the left of the radial line 9. The curvature angle δ therefore lies between a meridional tangent 7 which lies against the blade surface 6 on a leading edge 16 or on a trailing edge 12, and the radial line 9 which extends orthogonally to the rotational axis 8 of the turbomachine, and customarily has a value of 15°≦δ≦−20°.

According to the invention, the stator blade 4 also has a twist in the radial direction of the respective blade 4, which is shown in FIG. 3. The twist, or the twisting, in this case is defined by a metal angle α₂ which on the one hand is arranged between a circumferential line 21, which in the circumferential direction of the turbomachine connects the respective trailing edges 12 of the respective stator blades 4, and on the other hand the tangent to the curvature center line 13 at the leading edge 16 or at the trailing edge 12 respectively. Similarly to the sweep curvature or to the lean curvature, the metal angle α₂ also varies along the radial blade length, wherein in the region of the hub 2 the angle is larger than in the region of the casing 3. A range of the metal angle α₂ which is favorable for the aerodynamic conditions of the turbomachine in this case is customarily 25°≦α₂≦10°.

In FIG. 4, a longitudinal section in the region of the stator blade 4 through the turbomachine is shown, wherein a hub-side circumferential step 14 is to be seen, which in the direction of flow 15 falls away inwards and radially to the rotational axis 8 of the turbomachine. The circumferential step 14, according to the view in FIG. 4, has an S-shaped profile between the leading edge 16 and the trailing edge 12. This profile, however, is not compulsory, it can alternatively also have a linear progression between the leading edge 16 and trailing edge 12. As a result of the circumferential step 14, a hub diameter at the leading edge 16 is larger than at the trailing edge 12, as a result of which the aerodynamic properties are also positively influenced. A height of the circumferential step 14 in this case is determined by the angles β₁ and β₂ which are determined in each case between a tangent 7 to the circumferential step 14 on the one hand, and the rotational axis 8 of the turbomachine or a line parallel to it on the other hand, and customarily lies within a range of −20°≦β_(1,2)≦20°. In this case, the tangent 7 to the circumferential step 14 has its greatest gradient at a point of intersection 17 at which the said tangent 7, a center of gravity line 18 and the circumferential step 14 intersect. In the case of an S-shaped cross-sectional form of the circumferential step 14, the point of inflection of the circumferential step customarily also lies at the said point of intersection 17.

In FIG. 5, a pitch ratio t/s is shown, i.e. the quotient of blade spacing t in the circumferential direction between two adjacent stator blades 4 and the chord length s over the radial extent of the stator blade 4. Both the chord length s and the blade spacing t in this case are recorded as linear values and can vary over the radial extent of the stator blade 4, wherein the pitch ratio t/s is customarily smaller at the blade root 2 than at the blade tip 3. A range within which the pitch ratio t/s customarily lies in this case is defined between 0.45≦t/s≦0.75.

In the view in FIG. 6, two further special features of the stator blades 4 according to the invention are shown, specifically an incidence angle α₁ which varies over the radial blade length of the stator blade 4 on the one hand, and also a wedge angle WE which varies over the radial blade length between a surface tangent 7 a to a pressure side 19 and a surface tangent 7 b to a suction side 20 at the trailing edge 12 of the stator blade 4. In this case, the inflow-side incidence angle α₁ of the curvature center line 13 at the blade root 2 is smaller than at the blade tip 3 and for example lies within a range of 55°≦α₁≦110°. The incidence angle α₁ therefore increases from the blade root 2 towards the blade tip 3. In contrast, the wedge angle WE at the blade root 2 is larger than at the blade tip 3 and decreases preferably continuously from the blade root 2 in the direction of the blade tip 3. The wedge angle WE customarily lies within a range of 15°≦WE≦0°.

It is worthy of note in this case that the stator blades 4 are formed in such a way that at least the curvature angle γ of the lean curvature and/or the curvature angle δ of the sweep curvature do not change along the radial blade length provided that the angles are measured with regard to the curvature center line 13 or with regard to the leading edge 16.

According to FIG. 6, a narrowest flow cross section q between two adjacent stator blades 4 is defined, which shifts between hub 2 and casing 3 against the direction of flow 15. In other words, this means that the flow bottleneck q at the hub 2 of two adjacent stator blades 4 lies in the region of a trailing edge 12, while in the region of the casing 3 of two adjacent stator blades 4 the flow bottleneck lies more in the region of the leading edges 16.

An angle Δα is defined according to FIG. 6, by the tangent 7′ on the one hand and by the tangent 7″ on the other hand. The tangent 7′ lies against the suction side 20 of the trailing edge 12, while the tangent 7″ lies against the suction side 20 of the stator blade 4 and at the same time is oriented orthogonally to the flow bottleneck q. The angle Δα in this case decreases according to the invention from the hub 2 towards the casing 3 and is variable along the radial blade length. A typical range for the angle Δα in this case lies between −5°≦Δα≦15°.

LIST OF DESIGNATIONS

1 Flow space 2 Hub of the turbomachine 3 Radial outer wall/casing 4 Stator blade 6 Blade surface

7 Tangent

8 Rotational axis of the turbomachine 9 Radial line 10 Blade chord 11 Point of intersection 12 Trailing edge 13 Curvature center line 14 Hub contour 15 Direction of flow 16 Leading edge 17 Point of intersection 18 Center of gravity line 19 Pressure side of the stator blade 4 20 Suction side of the stator blade 4 21 Circumferential line α₁ Metal angle at the blade leading edge α₂ Metal angle at the blade trailing edge β Angle to the hub contour 14 γ Lean curvature angle δ Sweep curvature angle s Chord length t Blade spacing q Narrowest flow cross section WE Wedge angle 

1. A stator blade for a turbomachine, especially for a steam turbine, characterized by the following geometric features: a lean curvature perpendicular to the blade chord, essentially in the circumferential direction, a sweep curvature parallel to the blade chord, essentially in the axial direction of the turbomachine, a twist in the radial direction of the respective blade, a hub-side circumferential step, which in the direction of flow falls away inwards and radially to the rotational axis of the turbomachine, a chord length of the blade which varies over the radial extent of the stator blade, a cross-sectional profile of the blade which varies over the radial extent of the stator blade.
 2. The stator blade as claimed in claim 1, wherein the lean curvature varies along the radial blade length, and/or the lean curvature decreases along the radial blade length from the hub to the casing, and/or a curvature angle (γ) between a tangent which lies against the blade surface at a trailing edge or at a leading edge of the stator blade, and a radial line which extends orthogonally to the rotational axis of the turbomachine, lies within a range of 0°≦γ≦15°, and/or the stator blade has a positive lean curvature in the direction of rotation.
 3. The stator blade as claimed in claim 1, wherein the sweep curvature of the stator blade varies along the radial blade length, and/or the sweep curvature of the stator blade along the radial blade length has a positive value in the region of the hub and has a negative value in the region of the casing, and/or a curvature angle between a meridional tangent which lies against the blade surface at a leading edge or at a trailing edge, and a radial line which extends orthogonally to the rotational axis of the turbomachine, lies within a range of 15°≦δ≦−20°.
 4. The stator blade as claimed in claim 1, wherein a metal angle at the trailing edge is defined between a circumferential line in the circumferential direction of the turbomachine and a tangent to the curvature center line at the trailing edge, and/or the metal angle varies along the radial blade length, and/or the metal angle is larger at the hub than in the region of the casing, and/or the metal angle between a tangent to the curvature center line at the trailing edge of the stator blade and the rotational axis of the turbomachine lies with a range of 25°≦α₂≦100°.
 5. The stator blade as claimed in claim 1, wherein the hub-side circumferential step has an S-shaped profile between a leading edge and a trailing edge of the stator blade or extends linearly between the two edges, and/or the leading edge and the trailing edge do not extend in a parallel manner, and/or an angle between a tangent to the circumferential step and the rotational axis of the turbomachine lies within a range of −20°≦β≦20°.
 6. The stator blade as claimed in claim 1, wherein a pitch ratio, that is to say a quotient of a blade spacing between adjacent stator blades in the circumferential direction and a chord length varies over the radial extent of the stator blade, and/or the pitch ratio at the hub is smaller than in the region of the casing, and/or the pitch ratio lies within a range of 0.45≦t/s≦0.75.
 7. The stator blade as claimed in claim 1, wherein an inflow-side incidence angle of the curvature center line varies over the radial blade length of the stator blade, and/or the inflow-side incidence angle of the curvature center line is smaller at the hub than in the region of the casing, and/or the inflow-side incidence angle of the curvature center line lies within a range of 55°≦α₁≦110°.
 8. The stator blade as claimed in claim 1, wherein a wedge angle between a surface tangent to a pressure side and a surface tangent to a suction side at a trailing edge of the stator blade varies over the radial blade length of the stator blade, and/or the wedge angle is larger at the hub than in the region of the casing (3), and/or the wedge angle lies within a range of 15°≦WE≦0°.
 9. The stator blade as claimed in claim 1, wherein a narrowest flow cross section between adjacent stator blades shifts from the hub towards the casing against the direction of flow. 